DESCRIPTION (Adapted from applicant's description): The objectives of this application are to determine the mechanisms involved in X chromosome inactivation and to understand the impact of this process on human X-linked disorders. The mechanisms involved in determining which genes are expressed from the inactive X and which ones are silenced are currently unknown. The specific aims of this application are: (1) to determine whether there is a "domain" of genes on the mouse X chromosome that escapes inactivation; (2) to define the characteristics of the mouse X chromosome with respect to inactivation status; and (3) to determine whether the inactivation status of a gene is determined at the level of chromosomal domains or on an individual gene basis.